Different types of filters exist for separating solid particles from liquid in a form that is known as a filter cake as described in Chapter 11 of “Chemical Process Equipment Selection and Design” by Stanley M. Walas, the entire content of which is hereby incorporated by reference herein. Once formed, washing and drying is performed on the filter cake and the filter cake is discharged.
In general, the solid/liquid separation equipment currently available on the market comes in the following categories:
1. Vacuum Filters
2. Pressure Filters
3. Centrifuges
4. Thickeners
5. Clarifiers
Vacuum filters include vacuum drum filters and rotary belt filters, which provide for continuous, semi-continuous or batch operation using a moving filtration medium on a drum, disc or along a belt. The most popular pressure filters include filter presses, candle filters and plate filters, such as horizontal plate/leaf type filters, which provide only for batch operation.
A candle filter consists of a plurality of candles suspended in a pressure vessel. Each candle is elongate and circular in shape like a candle and consists of filtration media arranged around a core consisting of a bundle of perforated tubes. The slurry feed is pumped into the bottom of the pressure vessel and is passed under pressure through the filtration media such that liquid, known as filtrate, enters into the cores and is drawn out from the top while the solid particles remain on the filtration media and build up into a filter cake. The housing of the pressure vessel is under pressure supplied by an upstream feed pump. Once the filter cake reaches a certain thickness (˜5 cm), the flow of slurry into the housing is cut off and any remaining liquid is drained from the housing. The filter cake is dumped to the bottom of the housing by vibration or an air pulse applied backward inside the candles. Plate filters have a similar mode of operation, with a different shape of the filter elements.
While candle and plate filters are advantageous for different applications, the inventor has recognized a number of disadvantages. First, these pressure filters can be batch operated only. Second, cake washing and drying cannot be easily performed prior to cake discharge because pressure would need to be maintained in the housing to retain the filter cake when the pressure vessel is depressurized to be emptied from slurry. Also, even though the candles offer a large and effective filtration area for their, the entire size of a candle filter is limited by the size of the pressure vessel because pressure vessels are expensive and increase in cost with increasing diameter. As a result, candle filters have been mostly heretofore used for clarification and polishing.